Flip-chip bonding utilizes bumps to establish electrical contact between a chip's contact pads and a package substrate. Structurally, a bump structure actually contains a bump itself and a so-called under bump metallurgy (UBM) layer located between the bump and a contact pad. An UBM layer generally comprises a diffusion barrier layer (or a glue layer) and a seed layer, arranged in that order, on the contact pad. The bumps themselves, based on the material used, are classified as solder bumps, gold bumps, copper pillar bumps and bumps with mixed metals. Recently, copper pillar bump technology has been proposed. Compared with circuits using a solder bump, the circuit connected to a package substrate by a copper pillar bump has a finer pitch with minimum probability of bump bridging, reduces the capacitance load for the circuits, and allows the electronic component to perform at higher frequencies.
However, there are challenges to implementing such features and processes in IC fabrication. For example, poor adhesion of an electronic component to a package substrate due to oxidation of copper pillar. Accordingly, what is needed is a method for protecting a copper pillar.